The present invention relates to a reflective scanning optical system for scanning a laser beam on a scan target surface by dynamically deflecting the laser beam using a revolving polygon mirror and focusing the dynamically deflected laser beam on the scan target surface using an imaging optical system including a curved mirror.
A scanning optical system scans a laser beam in a main scanning direction on a scan target surface (e.g. outer circumferential surface of a photosensitive drum) generally by revolving a polygon mirror (having the shape of a polygonal prism) about its central axis and letting each lateral face of the revolving polygon mirror deflect the laser beam dynamically. When the scan target surface itself is moved in an auxiliary scanning direction which is orthogonal to the main scanning direction, a plurality of linear traces (scan lines) are formed on the scan target surface at even intervals. Therefore, if the laser beam has been ON-OFF modulated according to image information, a two-dimensional image is formed on the scan target surface.
In conventional scanning optical systems, an imaging optical system having an fθ lens group for the correction of scan speed is generally placed between the revolving polygon mirror and the scan target surface in order to let the laser beam (dynamically deflected by the polygon mirror revolving at a constant angular speed) be scanned on the scan target surface at a constant speed.
Recently, reflective scanning optical systems, employing a curved mirror instead of the fθ lens group of the imaging optical system, have been proposed, from which reduction of chromatic aberration and miniaturization of the system can be expected. The imaging optical system equipped with the curved mirror has the same functions (scan speed correction function, field curvature correction function and optical face angle error correction function) as the conventional imaging optical system equipped with the fθ lens group (see Japanese Patent Provisional Publication No. HEI07-191272. for example).
Incidentally, in generally-used scanning optical systems (including the reflective scanning optical systems), the optical path length of the laser beam before being focused on the scan target surface varies depending on the incident position of the laser beam on the imaging optical system measured in the main scanning direction. Therefore, the magnification of the imaging optical system in the auxiliary scanning direction might change depending on the incident position of the laser beam on the imaging optical system measured in the main scanning direction, by which the convergence angle of the laser beam in the auxiliary scanning direction (which corresponds to the NA (Numerical Aperture)) might vary as the laser beam is scanned. Since the convergence angle of the beam focusing on the scan target surface corresponds to the so-called F number, the variation in the convergence angle will hereinafter be called “F number variation”. The F number variation eventually causes a change in the spot diameter (diameter of a beam spot formed by the laser beam focusing on the scan target surface).
Further, in cases where such an imaging optical system is employed for a so-called multibeam optical system which dynamically deflects a plurality of beams simultaneously, not only the spot diameter but also the interval between the scan lines changes.
In order to resolve the above problems, the assignee of the present invention has proposed a reflective scanning optical system which can cancel out the change in the spot diameter caused by the F number variation by use of the field curvature (curvature of field, image surface curvature) in the auxiliary scanning direction (see Japanese Patent Provisional Publication No. HEI08-262323, for example). However, in such a reflective scanning optical system, an extremely high accuracy is required of the assembly process and even a slight assembly error might ruin expected effects.
Therefore, the assignee has further proposed a reflective scanning optical system capable of reducing the change in the magnification of the imaging optical system in the auxiliary scanning direction and the F number variation accompanying the change by use of a curved mirror having a particular surface shape whose refractive power in the auxiliary scanning direction changes depending on the position in the main scanning direction (see Japanese Patent Provisional Publication No.HEI10-54952, for example). By such a reflective scanning optical system, the spot diameter on the scan target surface can be maintained constant, and in multibeam scanning optical systems, the interval between the scan lines on the scan target surface can also be made even.
However, employing a complex surface shape for the curved mirror as above drives up manufacturing cost of the curved mirror, increasing the total cost of the reflective scanning optical system. For this reason, the employment of the imaging optical system having the curved mirror of the complex surface shape has been practically difficult.